Using non-carbon anodes—i.e. anodes which are not made of carbon as such, e.g. graphite, coke, etc. . . . , but possibly contain carbon in a compound—for the electrowinning of aluminium should drastically improve the aluminium production process by reducing pollution and the cost of aluminium production. Many attempts have been made to use oxide anodes, cermet anodes and metal-based anodes for aluminium production, however they were never adopted by the aluminium industry.
For the dissolution of the raw material, usually alumina, a highly aggressive fluoride-based electrolyte, such as cryolite, is required.
Materials for protecting aluminium electrowinning components have been disclosed in U.S. Pat. Nos. 5,310,476, 5,340,448, 5,364,513, 5,527,442, 5,651,874, 6,001,236, 6,287,447 and in PCT publication WO01/42531 (all assigned to MOLTECH). Such materials are made predominantly (more than 50%) of non-oxide ceramic materials, e.g. borides, carbides or nitrides, for exposure to molten aluminium and to a molten fluoride-based electrolyte and have successfully been used in cathode applications. However, these non-oxide ceramic-based materials do not resist immediate exposure to anodically produced nascent oxygen.
The materials having the greatest resistance to oxidation are metal oxides which are all to some extent soluble in cryolite. Oxides are also poorly electrically conductive, therefore, to avoid substantial ohmic losses and high cell voltages, the use of non-conductive or poorly conductive oxides should be minimal in the manufacture of anodes. Whenever possible, a good conductive material should be utilised for the anode core, whereas the surface of the anode is preferably made of an oxide having a high electrocatalytic activity.
Several patents disclose the use of an electrically conductive metal anode core with an oxide-based active outer part, in particular U.S. Pat. Nos. 4,956,069, 4,960,494, 5,069,771 (all Nguyen/Lazouni/Doan), U.S. Pat. No. 6,077,415 (Duruz/de Nora), U.S. Pat. No. 6,103,090 (de Nora), U.S. Pat. No. 6,113,758 (de Nora/Duruz) and U.S. Pat. No. 6,248,227 (de Nora/Duruz), U.S. Pat. No. 6,361,681 (de Nora/Duruz), U.S. Pat. No. 6,365,018 (de Nora), U.S. Pat. No. 6,372,099 (Duruz/de Nora), U.S. Pat. No. 6,379,526 (Duruz/de Nora), U.S. Pat. No. 6,413,406 (de Nora), U.S. Pat. No. 6,425,992 (de Nora), U.S. Pat. No. 6,436,274 (de Nora/Duruz), U.S. Pat. No. 6,521,116 (Duruz/de Nora/Crottaz), U.S. Pat. No. 6,521,115 (Duruz/de Nora/Crottaz), U.S. Pat. No. 6,533,909 (Duruz/de Nora), U.S. Pat. No. 6,562,224 (Crottaz/Duruz) as well as PCT publications WO00/40783 (de Nora/Duruz), WO01/42534 (de Nora/Duruz), WO01/42535 (Duruz/de Nora), WO01/42536 (Nguyen/Duruz/de Nora), WO02/070786 (Nguyen/de Nora), WO02/083990 (de Nora/Nguyen), WO02/083991 (Nguyen/de Nora), WO03/014420 (Nguyen/Duruz/de Nora), WO03/078695 (Nguyen/de Nora), WO03/087435 (Nguyen/de Nora).
U.S. Pat. No. 4,374,050 (Ray) discloses numerous multiple oxide compositions for electrodes. Such compositions inter-alia include oxides of iron and cobalt. The oxide compositions can be used as a cladding on a metal layer of nickel, nickel-chromium, steel, copper, cobalt or molybdenum.
U.S. Pat. No. 4,142,005 (Cadwell/Hazelrigg) discloses an anode having a substrate made of titanium, tantalum, tungsten, zirconium, molybdenum, niobium, hafnium or vanadium. The substrate is coated with cobalt oxide Co3O4.
U.S. Pat. No. 6,103,090 (de Nora), U.S. Pat. No. 6,361,681 (de Nora/Duruz), U.S. Pat. No. 6,365,018 (de Nora), U.S. Pat. No. 6,379,526 (de Nora/Duruz), U.S. Pat. No. 6,413,406 (de Nora) and U.S. Pat. No. 6,425,992 (de Nora), and WO04/018731 (Nguyen/de Nora) disclose anode substrates that contain at least one of chromium, cobalt, hafnium, iron, molybdenum, nickel, copper, niobium, platinum, silicon, tantalum, titanium, tungsten, vanadium, yttrium and zirconium and that are coated with at least one of ferrites of cobalt, copper, chromium, manganese, nickel and zinc. WO01/42535 (Duruz/de Nora) and WO02/097167 (Nguyen/de Nora), disclose aluminium electrowinning anodes made of surface oxidised iron alloys that contain at least one of nickel and cobalt. U.S. Pat. No. 6,638,412 (de Nora/Duruz) discloses the use of anodes made of a transition metal-containing alloy having an integral oxide layer, the alloy comprising at least one of iron, nickel and cobalt.
Metal-based anodes are liable to corrosion and/or passivation in aluminium electrowinning cells. To avoid of minimise such mechanism, the composition and temperature of the cell's electrolyte should be chosen accordingly.
WO00/06804 discloses that a nickel-iron anode may be used in an electrolyte at a temperature of 820° to 870° C. containing 23 to 26.5 weight % AlF3, 3 to 5 weight % Al2O3, 1 to 2 weight % LiF and 1 to 2 weight % MgF2.
U.S. Pat. Nos. 5,006,209 and 5,284,562 (both Beck/Brooks), U.S. Pat. No. 6,258,247 and U.S. Pat. No. 6,379,512 (both Brown/Brooks/Frizzle/Juric), U.S. Pat. No. 6,419,813 (Brown/Brooks/Frizzle) and U.S. Pat. No. 6,436,272 (Brown/Frizzle) all disclose the use of nickel-copper-iron anodes in an aluminium production electrolyte at 660°-800° C. containing 6-26 weight % NaF, 7-33 weight % KF, 1-6 weight % LiF and 60-65 weight % AlF3. The electrolyte may contain Al2O3 in an amount of up to 30 weight %, in particular 5 to 10 or 15 weight %, most of which is in the form of suspended particles and some of which is dissolved in the electrolyte, i.e. typically 1 to 4 weight % dissolved Al2O3. In U.S. Pat. Nos. 6,258,247, 6,379,512, 6,419,813 and 6,436,272 such an electrolyte is said to be useable at temperatures up to 900° C. In U.S. Pat. Nos. 6,258,247 and 6,379,512 the electrolyte further contains 0.004 to 0.2 weight % transition metal additives to facilitate alumina dissolution and improve cathodic operation.
U.S. Pat. No. 5,725,744 (de Nora/Duruz) discloses an aluminium production cell having anodes made of nickel, iron and/or copper in a electrolyte at a temperature from 680° to 880° C. containing 42-63 weight % AlF3, up to 48 weight % NaF, up to 48 weight % LiF and 1 to 5 weight % Al2O3. MgF2, KF and CaF2 are also mentioned as possible bath constituents.
WO2004/035871 (de Nora/Nguyen/Duruz) discloses a metal-based anode containing at least one of nickel, cobalt and iron. The anode is used for electrowinning aluminium in a fluoride-containing molten electrolyte consisting of: 5 to 14 wt % dissolved alumina; 35 to 45 wt % aluminium fluoride; 30 to 45 wt % sodium fluoride; 5 to 20 wt % potassium fluoride; 0 to 5 wt % calcium fluoride; and 0 to 5 wt % of further constituents.
Non-carbon anodes have not as yet been commercially and industrially applied and there is still a need for a metal-based anodic material that can be used in an appropriate electrolyte for electrowinning aluminium.